This means that when you iterate over the array using a simple for loop, no other code is running, if a user takes an action that will change the array, the corresponding request will be handled only after you have finished.
The downside to this is of course that your users will experience a delay whenever you are doing an update. But this is not a problem unless your update takes a long time.
Beware that any code you put in a callback will be executed under a different event than the current code, so arbitrary other events may be handled in-between.
On a slightly related note, beware that the
setInterval functions are not exact in their timing. In V8 an interval will "drift", taking breaks that are slightly longer than specified, or in some cases under heavy load a lot longer. If you want your updates to stick to a schedule you will have to use
setTimeout, and in each iteration check how much off schedule it was executed and subtract that from the time set for the next update.